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total, counting the one around the base) and that we measure the highest point at

the top of the hill to be 46 feet high, marking it with a flag.

46'

30

40 feet BM 46 0
30 feet 10

20 feet 40
20
10 feet

0 feet

Profile (side) view Map (top) view

Fig. 4.1 Contour lines drawn on an imaginary hill viewed from the side and top

Now, if we flew over the hill in a helicopter and photographed it from above,
the lines we have marked on the hill (side view, Figure 4.1) would appear as a
series of concentric circles (top view, Figure 4.1). We could then mark each circle
in the photograph with the height that it represents, and also mark the summit of
the hill with the height that we measured.

Having done this, we have constructed a topographic contour map. Each
line is called a contour line, and the change in height from one line to the next
(10 feet in this case) is called the contour interval. The measured height marked

at the top of the hill we call a bench mark (BM). The base of the hill where we
began measuring elevation is our zero elevation point or datum. In most cases,
datum on topographic maps is defined as mean sea level.
Contour lines also indicate the slope of the Earth's surface. Where contour

lines are closely spaced, slopes are steep. Where the contour lines are spaced
widely apart, slopes are gentle. All of the land on one side of a contour is higher
than the land on the other side of the contour. Therefore, when you cross a
contour, you either go uphill or downhill. The basic determination in reading any

contour map is to figure out the direction of slope of the land. Careful
examination of stream-flow directions and bench marks will give you a general
feeling for the overall slope of the Earth's surface in any given map area. Some
general rules for contour lines are given below:

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